ERR_EMPTY_RESPONSE When Trying to Login on Client’s Host

I’m having issues with a client’s host. I’ve never seen this problem in the 6 years I’ve been working with WordPress, but hopefully someone else has!

I can’t login to the website. I can go to the login page and that loads just fine. However, when I try to login, I get an “ERR_EMPTY_RESPONSE” error in Chrome. I’ve tried it in Firefox and Safari with similar results. I’ve cleared my browser cache, tested on different networks, and had other people on other computers test it. Same result every time.

The weirdest part is that the client doesn’t have an issue logging in. I *think* if they clear their browser cache, they may no longer be able to login but I haven’t asked them to do that yet.

I was able to login before. This error popped up the last time I migrated the database from my development environment. I thought I messed up something with the database, but the front-end is working just fine.

The website is hosted here: https://telmex.com/

Any ideas are most appreciated!

The page I need help with: [log in to see the link]